Fix properties being lost when reloading placeholder GDScript instance

During reloading in `GDScriptLanguage::reload_all_scripts` a placeholder instance that must remain so is replaced with a new placeholder instance. The state is then restored by calling `ScriptInstance::set` for each property. This does not work if the script is missing the properties due to build/parse failing.
The fix for such cases is to call `placeholder_set_fallback` instead of `set` on the script instance.

I took this chance to move the `build_failed` flag from `PlaceHolderScriptInstance` to `Script`. That improves the code a lot. I also renamed it to `placeholder_fallback_enabled` which is a much better name (`build_failed` could lead to misunderstandings).

2 files changed, 27 insertions, 15 deletions

diff --git a/modules/gdscript/gdscript.cpp b/modules/gdscript/gdscript.cpp
index 538249c8e2..3904ec6389 100644
--- a/modules/gdscript/gdscript.cpp
+++ b/modules/gdscript/gdscript.cpp
@@ -475,20 +475,15 @@ bool GDScript::_update_exports() {
 				_signals[c->_signals[i].name] = c->_signals[i].arguments;
 			}
 		} else {
-			for (Set<PlaceHolderScriptInstance *>::Element *E = placeholders.front(); E; E = E->next()) {
-				E->get()->set_build_failed(true);
-			}
-			return false;
-		}
-	} else {
-		if (!valid) {
-			for (Set<PlaceHolderScriptInstance *>::Element *E = placeholders.front(); E; E = E->next()) {
-				E->get()->set_build_failed(true);
-			}
+			placeholder_fallback_enabled = true;
 			return false;
 		}
+	} else if (!valid || placeholder_fallback_enabled) {
+		return false;
 	}
 
+	placeholder_fallback_enabled = false;
+
 	if (base_cache.is_valid()) {
 		if (base_cache->_update_exports()) {
 			changed = true;
@@ -503,7 +498,6 @@ bool GDScript::_update_exports() {
 		_update_exports_values(values, propnames);
 
 		for (Set<PlaceHolderScriptInstance *>::Element *E = placeholders.front(); E; E = E->next()) {
-			E->get()->set_build_failed(false);
 			E->get()->update(propnames, values);
 		}
 	}
@@ -907,6 +901,7 @@ GDScript::GDScript() :
 	tool = false;
 #ifdef TOOLS_ENABLED
 	source_changed_cache = false;
+	placeholder_fallback_enabled = false;
 #endif
 
 #ifdef DEBUG_ENABLED
@@ -1675,6 +1670,8 @@ void GDScriptLanguage::reload_tool_script(const Ref<Script> &p_script, bool p_so
 		//restore state if saved
 		for (Map<ObjectID, List<Pair<StringName, Variant> > >::Element *F = E->get().front(); F; F = F->next()) {
 
+			List<Pair<StringName, Variant> > &saved_state = F->get();
+
 			Object *obj = ObjectDB::get_instance(F->key());
 			if (!obj)
 				continue;
@@ -1684,16 +1681,26 @@ void GDScriptLanguage::reload_tool_script(const Ref<Script> &p_script, bool p_so
 				obj->set_script(RefPtr());
 			}
 			obj->set_script(scr.get_ref_ptr());
-			if (!obj->get_script_instance()) {
+
+			ScriptInstance *script_instance = obj->get_script_instance();
+
+			if (!script_instance) {
 				//failed, save reload state for next time if not saved
 				if (!scr->pending_reload_state.has(obj->get_instance_id())) {
-					scr->pending_reload_state[obj->get_instance_id()] = F->get();
+					scr->pending_reload_state[obj->get_instance_id()] = saved_state;
 				}
 				continue;
 			}
 
-			for (List<Pair<StringName, Variant> >::Element *G = F->get().front(); G; G = G->next()) {
-				obj->get_script_instance()->set(G->get().first, G->get().second);
+			if (script_instance->is_placeholder() && scr->is_placeholder_fallback_enabled()) {
+				PlaceHolderScriptInstance *placeholder = static_cast<PlaceHolderScriptInstance *>(script_instance);
+				for (List<Pair<StringName, Variant> >::Element *G = saved_state.front(); G; G = G->next()) {
+					placeholder->property_set_fallback(G->get().first, G->get().second);
+				}
+			} else {
+				for (List<Pair<StringName, Variant> >::Element *G = saved_state.front(); G; G = G->next()) {
+					script_instance->set(G->get().first, G->get().second);
+				}
 			}
 
 			scr->pending_reload_state.erase(obj->get_instance_id()); //as it reloaded, remove pending state
diff --git a/modules/gdscript/gdscript.h b/modules/gdscript/gdscript.h
index 752d660ffb..71282bcdc8 100644
--- a/modules/gdscript/gdscript.h
+++ b/modules/gdscript/gdscript.h
@@ -97,6 +97,7 @@ class GDScript : public Script {
 	Ref<GDScript> base_cache;
 	Set<ObjectID> inheriters_cache;
 	bool source_changed_cache;
+	bool placeholder_fallback_enabled;
 	void _update_exports_values(Map<StringName, Variant> &values, List<PropertyInfo> &propnames);
 
 #endif
@@ -209,6 +210,10 @@ public:
 	virtual void get_constants(Map<StringName, Variant> *p_constants);
 	virtual void get_members(Set<StringName> *p_members);
 
+#ifdef TOOLS_ENABLED
+	virtual bool is_placeholder_fallback_enabled() const { return placeholder_fallback_enabled; }
+#endif
+
 	GDScript();
 	~GDScript();
 };